1. Field of the Invention
The present invention relates generally to a method of accessing a UTRAN (UMTS Terrestrial Radio Access Network) by a UE (User Equipment) in UMTS (Universal Mobile Telecommunication System), and in particular, to a method of reducing access time to a UTRAN.
2. Description of the Related Art
In general, a UE transmits only a preamble to a UTRAN at an access attempt. Upon receipt of the preamble, the UTRAN is synchronized to the UE and notifies the UE that the synchronization is completed via an AICH (Acquisition Indication Channel).
In UMTS, the UE uses an RACH (Random Access Channel) to access the UTRAN. When attempting a call, the UE transmits an RRC (Radio Resource Control) Connection message to the UTRAN on the RACH. Besides, the RACH is used for cell update, transmission of a URA (UTRAN Registration Area) update message, and transmission of a very small amount of data in a Cell_FACH state, in the case where the UE is connected to the UTRAN. Now a brief description of the RACH will be given below with respect to FIG. 1.
FIG. 1 is a flowchart illustrating a conventional UTRAN operation after receiving a preamble signature from a UE.
Information about available sub-channels and available preamble signatures according to ASC (Access Service Class) is included in PRACH (Physical RACH) information (or PRACH information). According to the PRACH information, a preamble signature is assigned to a UE depending on what ASC the UE intends to use on an available RACH sub-channel. The preamble signature is determined uniformly, randomly based on the IMSI (International Mobile Subscriber Identity) of the UE and time information. After the preamble signature is determined, access slots for the preamble are determined. When there are no available access slots, the next slot set is considered. Then, the UE transmits to the UTRAN the preamble with the determined signature in the access slots with predetermined power.
Upon receipt of the preamble signature in step 100, the UTRAN checks for the signatures in step 110, and the UTRAN determines whether an RACH corresponding to the preamble signature is in use in step 120. If the RACH is not used, the UTRAN transmits the AICH to the UE. On the other hand, if the RACH is in use, the UTRAN transmits a negative AICH to the UE in step 130.
Meanwhile, if the UE fails to receive the AICH from the UTRAN, it determines next access slots, a preamble signature uniformly random, and retransmits the preamble signature with power increased by a power ramp step. In case of negative AICH, the UE terminates the access procedure. In this case, since another UE is using the current preamble signature, the UE re-attempts an access after a back-off delay (TB01=NB01×10 ms). Here, minimum and maximum values in RACH transmission parameters of a BCH are received as NB01.
In case of positive AICH, the UE transmits a RACH message with power increased by Pp-m[dB] from the last preamble to the UTRAN.
In the above-described manner, the RACH is used in UMTS. The UTRAN transmits a negative AICH to a UE when different UEs including the UE use the current signature for an RACH or access using different signatures corresponding to the same RACH in the same access slot. Upon receipt of the negative AICH, the UE re-initiates the RACH procedure after a back-off delay (TB01=NB01×10 ms).
Even if the system supports a plurality of RACHs and there remain available RACHs, the unnecessary access delay occurs when preamble signatures collide or signatures corresponding to the same RACH are used when accessing because no determination is made as to availability of remaining RACHs.